DFC,
I was on Lamictal for almost 2 years in the past, and was on a dose of 300mg in the end. I used to get the brain fogs, but not as much as the ones I get on Lithium. But it can also be your meds combo that give you the fogs.
I used to also get random, but minor rashes/hives. That is one of side effects of Lamictal. I ran to my pdoc and other specialists and they told me it was minor rashes and nothing to worry about. I was told that if it was Stevens-Johnson that I would know. I was like you when I was on Lamictal, so I went looking in medical books what it looked like and it was very scary.
I was also told that, you would have a high fever, vomiting and other major symptoms.
I noticed it was worse at times when I spent outdoors on a warmer day or when I was out in the sun. It actually tells you on the medication info sheet not to be out in the sun too long.
http://us.gsk.com/products/assets/us_lamictal.pdf pages 35-37 have the most common side effects
But if you really are concerned, you should call you pdoc or a dermatologist and have it checked out.